Vehicle Communication Networks & Multiplexing

Section 4.5: Vehicle Communication Networks & Multiplexing

Modern vehicles contain dozens of electronic control modules that must share real-time data to coordinate vehicle operation. To eliminate the weight, complexity, and cost of routing dedicated point-to-point wires between every module, automotive manufacturers utilize multiplexing. Multiplexing is the process of transmitting multiple digital messages or data streams simultaneously over a single, shared communication medium (a bus network). This network architecture allows modules, such as the engine control module and transmission control module, to share sensor data like throttle position and engine speed instantly.

Network Topologies and Architectures

Automotive communication networks are arranged in specific physical layouts, known as topologies:

• Ring Topology: Modules are connected in a closed loop, where data passes sequentially from one module to the next. A failure in one module or wire breaks the entire loop. This is primarily used in fiber-optic infotainment systems.
• Star Topology: All modules connect to a central splice pack or gateway module. If one module or its wire fails, the rest of the network remains operational. However, a failure of the central gateway disables the entire network.
• Bus Topology: A single main trunk line running through the vehicle, with individual modules connected via short branch lines (stub wires). This is the standard layout for powertrain and chassis networks.

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Controller Area Network (CAN)

The Controller Area Network (CAN) is the dominant multiplexed communication standard in modern light-duty vehicles. Developed by Bosch and standardized globally, high-speed CAN (CAN-C) operates at a transmission speed of 500 kilobits per second (kbps), while low-speed CAN (CAN-B) operates at 125 kbps.

1. Physical Layer and Differential Voltage Signaling

High-speed CAN uses a two-wire bus consisting of a CAN High (CAN-H) and a CAN Low (CAN-L) wire. These wires are twisted together (twisted-pair) to cancel out electromagnetic interference (EMI) and radio frequency interference (RFI).

CAN uses differential voltage signaling, meaning the network logical states (zeros and ones) are determined by the difference in voltage between CAN-H and CAN-L, rather than a single wire's voltage relative to ground. This design makes the network highly resistant to electrical noise:

• Recessive State (Logical 1): When no module is transmitting a dominant bit, the bus rests in the recessive state. Both CAN-H and CAN-L are biased to approximately 2.5 volts. The differential voltage (CAN-H minus CAN-L) is 0 volts.
• Dominant State (Logical 0): When a module transmits a bit, it drives the bus to the dominant state. CAN-H is pulled up to approximately 3.5 volts, and CAN-L is pulled down to approximately 1.5 volts. The differential voltage is 2.0 volts.

Because the receiver circuits only measure the difference between the two wires, any external noise spike that affects both wires equally will not alter the differential voltage, preventing data corruption.

2. Termination Resistors

To prevent electrical signal reflections (echoes) from bouncing back from the ends of the bus wires and corrupting data, high-speed CAN networks require termination resistors. The network uses two 120-ohm resistors connected in parallel across the CAN-H and CAN-L lines at the physical extremities of the bus (typically inside the Engine Control Module - ECM and the Instrument Cluster or Body Control Module - BCM).

By applying Ohm's law for parallel resistance:

Total Resistance = 1 / ((1 / 120) + (1 / 120)) = 60 ohms

Technicians can verify network integrity by connecting a digital multimeter (DMM) across Data Link Connector (DLC) Pin 6 (CAN-H) and Pin 14 (CAN-L). With the battery disconnected:

• A reading of 60 ohms indicates both termination resistors and the bus wiring are intact.
• A reading of 120 ohms indicates one of the termination resistors or its associated branch circuit is open.
• A reading of near 0 ohms indicates a short circuit between the CAN-H and CAN-L wires.
• An infinite resistance reading indicates an open circuit in both terminating paths or a complete break in the bus trunk lines.

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Local Interconnect Network (LIN)

The Local Interconnect Network (LIN) is a low-speed, single-wire sub-network designed to reduce cost in non-critical applications. LIN operates at speeds up to 20 kbps. It uses a master-slave configuration, where a single master module (such as the BCM) controls all communication on the LIN bus. The slave modules (such as window switches, power mirrors, wiper motors, or climate controls) only transmit data when specifically commanded by the master. LIN does not require termination resistors and is highly cost-effective because it uses a single unshielded wire. The master module acts as a gateway module, translating LIN commands to CAN messages so they can be read by other high-speed modules.

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Specialty Networks

• Media Oriented Systems Transport (MOST): A high-speed network that uses fiber-optic cables to transmit audio, video, and infotainment data in a ring topology at speeds up to 150 Mbps. Instead of electrical current, MOST transmits pulses of light. If a fiber-optic cable is bent too sharply, kinked, or scratched, light transmission fails, and the entire infotainment loop goes down.
• FlexRay: An ultra-high-speed, time-triggered, redundant protocol (up to 10 Mbps) used for safety-critical drive-by-wire applications, active suspension, and advanced driver assistance systems (ADAS) where immediate, deterministic timing is required.

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Diagnostic Procedures for Communication Networks

When a network fault occurs, modules cannot exchange data, generating a diagnostic trouble code (DTC) starting with the letter "U" (U-codes), such as U0100 (Lost Communication with ECM). Network diagnosis must follow a structured path:

1. Verify Battery Voltage: Low system voltage is the leading cause of communication faults. If system voltage drops below 10.5V, modules may randomly shut down or fail to boot.
2. Perform a Network Scan: Connect a scan tool to the DLC and execute a global module scan to determine which modules are communicating and which are offline.
3. Measure Resistance at DLC: Disconnect the vehicle battery. Set a DMM to ohms and measure between Pin 6 (CAN-H) and Pin 14 (CAN-L). A reading of 60 ohms is normal. If it is 120 ohms, locate the open circuit or the module containing the missing termination resistor.
4. Measure Bias Voltages: Reconnect the battery, turn the ignition key ON (engine off). Measure voltage from DLC Pin 6 to Pin 5 (Signal Ground). It should read approximately 2.5V to 2.7V. Measure DLC Pin 14 to Pin 5. It should read approximately 2.3V to 2.5V. The sum of the two voltages should be exactly 5.0V. If CAN-H reads 0V, there is a short to ground. If it reads 12V, there is a short to power.
5. Use a Digital Storage Oscilloscope (DSO): Connect channel 1 to CAN-H and channel 2 to CAN-L. Set the time base to 10–20 microseconds. You should see mirrored waveforms. Any spikes, noise, or flatlines indicate physical wiring damage, electromagnetic interference, or a failing transceiver inside a module.
6. Isolate Modules: If the bus is shorted, disconnect modules one by one while monitoring DLC resistance or the oscilloscope waveform. When the short disappears, the disconnected module is the source of the fault.